1. Field of the Invention
The present invention relates in general to a non-volatile memory, and more particularly, to a silicon nitride (SiN) read only memory (ROM) and the method of fabricating the same.
2. Description of the Related Art
The electrically erasable programmable read only memory (EEPROM) is a non-volatile memory that provides multiple data saving, reading, and erasing operations. The saved data will not disappear because of power interruption. Therefore, it has been broadly applied in personal computer and electronic equipment.
The typical EEPROM uses doped polysilicon to form a floating gate and a control gate. The electrons injected into the floating gate are uniformly distributed all over the floating gate. However, if a defect exists in the tunneling oxide layer under the floating gate, current leakage easily occurs affecting reliability of the device.
To resolve the EEPROM current leakage problem, a charge capture layer is used to replace the polysilicon floating gate. The material of the charge capture layer is silicon nitride. The silicon nitride charge capture layer normally has one silicon oxide layer on both sides of the silicon nitride layer stacked together to form a stacked gate structure of silicon oxide/silicon nitride/silicon oxide (ONO). The EEPROM including such stacked gate structure is typically referred to as the silicon nitride read only memory (NROM). When a voltage is applied to the control gate and the source/drain region for programming, hot electrons are generated in the channel near the drain region and injected to the charge capture layer. As silicon nitride has the electron capture characteristic, the electrons will not be uniformly distributed all over the charge capture layer, but are concentrated in a local area of the charge capture layer. As the electrons are injected into and trapped in the local area of the charge capture layer, the device is thus less sensitive to a defect of the tunneling oxide layer, such that leakage current is less likely to occur.
The other programming advantage of the NROM includes storing electrons in the silicon nitride layer at the side of the source/drain region by supplying higher voltage at the source/drain region at one side of the stacked gate. On the other hand, electrons can also be saved in the silicon nitride layer near the source/drain region at the other side of the stacked gate. Therefore, by adjusting the voltage applied to the source/drain regions at two sides of the control gate, the single silicon layer can store two groups of electrons, a single of electrons or no electrons. Therefore, four states can be written in a single memory cell to function as a non-volatile memory cell with 2 bits per cell.
However, while programming the conventional 2-bit NROM, the hot electrons injected into the electron capture layer are distributed according to the injection energy. The two bits of the same memory cell are mutually affected by each other to cause the electron secondary effect to link the charge distribution curves thereof. Therefore, during erase operation, the distribution curves of the hot holes injected into the charge capture layer cannot be superimposed with the electron distribution curves causing incomplete erase, or a longer time to erase.
Further, while using the hot electron injection to perform the erase operation, holes are injected into the charge capture layer via the drain side (or source side) and the number thereof are difficult to control. Therefore, excessive or insufficient holes may be injected to the charge capture layer causing over erase or insufficient erase of the memory device. When the over erase or the insufficient erase is serious, reliability of the memory device may be reduced.
In addition, in the conventional NROM, the material of the gate dielectric layer between the control gate and the charge capture layer is silicon oxide. The silicon oxide has a poor isolation effect that is likely to cause contact between the control gate and the charge capture layer, so as to affect reliability of the device.